1. Field of the Invention
The present invention relates to a clutch including an engaging element enabling the wedge engagement in the circumferential direction between an input member to which a rotational torque is input and a driven member. An operation state of the clutch is switchable between a state where the rotational torque is transmitted from the input member to the driven member and a state where the driven member freely rotates with respect to the input member. Such a clutch is applicable to, for example, a mechanism for switching an electrically-operated sliding door between an automatic opening/closing state and a manual opening/closing state.
2. Description of the Related Art
Conventionally, in an electrically-operated sliding door, the input of a rotational torque from a motor (driving device) drives a driving mechanism for a door in a forward/reverse rotation direction to open/close the door. In the case where the sliding door is to be manually opened/closed when a power switch is OFF or during a blackout, the door is too heavy to manually open/close because a rotor of the motor rotates along with the door movement. In order to overcome this inconvenience, there conventionally exists an electrically-operated sliding door including an electromagnetic clutch between a sliding door and a motor. When the electromagnetic clutch is turned OFF with the electromagnetic clutch being interposed between the sliding door and the motor, the door can be manually opened/closed with a small force because the connection between the door and the motor is cancelled. Moreover, such a sliding door is convenient because the sliding door is automatically switched to a manual opening/closing state in the case of emergency such as a blackout.
On the other hand, for a reclining seat, the inventors of the present invention have proposed a clutch for switching a seat back between a locked state and a free state by the operation of a lever, as a mechanism capable of adjusting the angle of the seat back in a stepless manner. In this clutch, an engaging element housing space is formed, which is gradually enlarged from one narrow side toward the opposite side in the axial direction between an inner ring connected to an operating lever and an outer ring connected to the seat back. In this engaging element housing space, the clutch includes an engaging element (tapered roller) that is engaged in the circumferential direction. The clutch operates to move the engaging element to the narrower side or the larger side of the engaging element housing space that is gradually enlarged in the axial direction so as to switch the seat back between the locked state and the free state (see Japanese Patent Laid-Open Publication No. 2001-140926, called as “Patent Reference 1” hereinafter).
Since the above-described electromagnetic clutch utilizes a frictional force obtained by (contact load due to electromagnetic force x friction coefficient), the electromagnetic clutch is increased in its size as well as in its mass in order to transmit a rotational torque without causing any slip. Therefore, in many cases, the electromagnetic clutch cannot meet the requirements of reduction in size and weight as well as in cost. Moreover, since the electromagnetic clutch is required to be always electrically conducted so as to maintain a torque transmission state, the electromagnetic clutch is disadvantage in its increased running cost.
On the other hand, the clutch, which has been proposed by the inventors of the present invention in the Patent Reference 1 regarding the reclining seat, has the function of switching the driven member between the locked state and the free state. However, the clutch disclosed in the Patent Reference 1 has been proposed supposing its application to the use where a large torque acts, such as to a reclining seat. On the contrary, in order to use this clutch for the electrically-operated sliding door, it is required to ensure sufficient rigidity of such a clutch to be used in a site that is subjected to a small rotational torque. At the same time, its fabrication cost is needed to be reduced. In order to solve these problems, various modifications should be further effected.